Within high solar concentration plants we can distinguish Stirling disks, parabolic trough collectors and the technology discussed herein, central receiver technology.
Central receiver systems consist of a heliostat field, made up of mirrors with a large surface area (40-125 m2 per unit) called sun-tracking heliostats, which reflect the direct solar radiation incident upon one or several central receiver devices installed on the highest part of a very high tower. These receiver devices are usually found housed in cavities “excavated” in the tower itself.
Concentrated solar radiation heats a fluid inside the receiver, the thermal energy of which can subsequently be used to generate electricity.
At present, water/steam technology is that most frequently used in central receiver systems, using both saturated and superheated steam as a heat-transfer fluid.
Therefore, these types of thermoelectric solar energy tower plants require a location with guaranteed availability of two resources: high solar irradiance and sufficient water supply. In general, those areas with high irradiance indices are areas where water supply is limited. To this end, in the search for more efficient plants with solar receiver devices with the least possible water consumption, the envisaged invention is intended for recirculating and saving the greatest possible amount of water with the lowest possible in-plant electricity consumption.
At present, conventional thermal power generation plants operate in the following manner: the heliostats reflect the solar radiation towards the receiver devices installed on the highest part of the tower, whereupon said energy evaporates a fluid and the steam is pumped towards a turbine to produce electricity and, at the outlet of said turbine, water is recovered from the steam, which is still at a high temperature. To this end, the steam that exits the turbine is redirected towards a condenser. Mains water circulates through said condenser at a temperature lower than that of the steam, in such a manner that the steam releases its heat into the mains water, condensing and enabling pumping thereof in order to re-circulate it back towards the receiver device.
The mains water that circulates through the condenser to cool the steam flows out at a temperature higher than that at which it flowed thereinto. In order to re-use this water in the condenser circuit, we must lower the temperature thereof. To this end, cooling towers are used wherein circulation is forced by means of large fans which allow the circulation of air and heat exchange between said air and water. The temperature of the hot water coming from the condensation circuit is lowered in these cooling towers by transferring heat and matter to the air circulating therein.
A heat transfer medium called “fill pack” is used to improve the air/water contact. Water enters the tower through the upper part thereof and is evenly distributed over the fill pack using sprays. In this manner, optimum contact between water and atmospheric air is achieved.
The fill pack serves to increase water/air exchange time and surface area. Once the water/air contact has been achieved, heat from the water is released into the air. This is due to two mechanisms: heat transfer by convection and water-to-air steam transfer, with the consequent cooling of the water by evaporation.
In heat transfer by convection, heat flows towards the air that surrounds the water due to the difference in temperature between the two fluids.
The evaporation cooling rate is significant in cooling towers, approximately 90% being due to the diffusion phenomenon. When air comes into contact with water, a thin film of saturated moist air is formed on the film of water that descends through the fill pack. This is due to the fact that partial water vapour pressure on the film of air is greater than that of the moist air that circulates through the tower, whereupon water vapour is released (evaporation). This body of evaporated water extracts latent vaporisation heat from the liquid itself. Said latent heat is released into the air, cooling the water and raising air temperature.
These previously envisaged systems have several drawbacks such as the electricity consumption generated by the use of fans in the cooling towers and the high degree of water consumption required.
In order to reduce electricity consumption in conventional thermal plants, so-called natural-draught or hyperbolic draught cooling is used.
The air flowing through the natural-draught tower is mainly due to the difference in density between the cold inflowing air and warm outflowing air. The air expelled by the column is lighter than the ambient air and a draught is created by means of the chimney effect, thereby eliminating the need for mechanical fans.
The difference in speeds between the wind circulating at ground level and the wind circulating through the highest part of the chimney also help to establish the air flow. For both reasons, natural-draught towers must be high and must also have a large cross-section to facilitate the movement of ascendant air. These towers have low maintenance costs and are highly recommended for cooling large quantities of water. The average speed of the air flowing through the tower is usually comprised between 1-2 m/s. In these types of natural-draught towers, highly compact “fill packs” are not used, due to the fact that airflow resistance must be as small as possible.
As mentioned earlier, these towers are commonly used in thermal plants, where tower construction requires considerable investment but is compensated with less electricity consumption.
With regard to water consumption, it cannot be reduced, due to which these types of plants are normally located in areas with a guaranteed water supply.